Ok, since I don't pay a lot of attention to color, being in the 5% that are red/green colorblind (only for nuanced colors, I am just fine for traffic lights), is it the case that the Sigma has its own "look", and the Sony doesn't??
Seems like changing cameras would always show some differences, although I have seen raves about the Sigma colors, I notice colors are different with different lenses, at least they seem that way to me.
*Waves* Hello from another of the 5%. Mine seems to be getting worse every year.
Due to how the Foveon sensor works, it responds to light completely different. The basic is that a bayer array has filters that allow each pixel to get red, green, or blue (I forget if white is also in the array) then uses it's neighbors to estimate what a pixel should be in full RGB response. A Foveon, by my understanding uses three layers of pixels stacked on top of each other, one blue, one red, and one green. This means each "pixel" is the composite of the stack, leading to hypothetically more accurate response to the collected light.
In a lab, this may be very true, but as a partially color blind person, you can agree that not all eyes respond the same, let alone how our brains view a scene.
As to the OP, have you tried to isolate what kind of conditions give you these "off" colors? Is it a certain color? Certain type of light?